Motor Cooling System Utilizing Axial Coolant Channels

ABSTRACT

An electric motor cooling system is provided that utilizes stator-integrated axial coolant channels and a coolant manifold centrally located within the stator to efficiently remove motor assembly heat. In order to increase the velocity of the coolant exiting the axial coolant channels, thereby improving end winding cooling uniformity, end laminations are integrated into the stator which restrict the flow of coolant from the axial coolant channels.

FIELD OF THE INVENTION

The present invention relates generally to the electric motor assemblyof an electric vehicle and, more particularly, to an efficient motorcooling system that can be used to cool the critical elements of a motorassembly.

BACKGROUND OF THE INVENTION

In response to the demands of consumers who are driven both byever-escalating fuel prices and the dire consequences of global warming,the automobile industry is slowly starting to embrace the need forultra-low emission, high efficiency cars. While some within the industryare attempting to achieve these goals by engineering more efficientinternal combustion engines, others are incorporating hybrid orall-electric drive trains into their vehicle line-ups. To meet consumerexpectations, however, the automobile industry must not only achieve agreener drive train, but must do so while maintaining reasonable levelsof performance, range, reliability, safety and cost.

The most common approach to achieving a low emission, high efficiencycar is through the use of a hybrid drive train in which an internalcombustion engine (ICE) is combined with one or more electric motors.While hybrid vehicles provide improved gas mileage and lower vehicleemissions than a conventional ICE-based vehicle, due to their inclusionof an internal combustion engine they still emit harmful pollution,albeit at a reduced level compared to a conventional vehicle.Additionally, due to the inclusion of both an internal combustion engineand an electric motor(s) with its accompanying battery pack, the drivetrain of a hybrid vehicle is typically more complex than that of eithera conventional ICE-based vehicle or an all-electric vehicle, resultingin increased cost and weight. Accordingly, several vehicle manufacturersare designing vehicles that only utilize an electric motor, or multipleelectric motors, thereby eliminating one source of pollution whilesignificantly reducing drive train complexity.

In order to achieve the desired levels of performance and reliability inan electric vehicle, it is critical that the temperature of the tractionmotor remains within its specified operating range regardless of ambientconditions or how hard the vehicle is being driven. A variety ofapproaches have been used to try and adequately cool the motor in anelectric car. For example, U.S. Pat. No. 6,954,010 discloses a devicesuch as a motor, transformer or inductor that utilizes a stack oflaminations, where a plurality of at least partially coincidentapertures pass through the stack of laminations and define a pluralityof coolant passageways. Manifold members located at opposite ends of thelamination stack are used to couple the coolant passageways to asuitable coolant pump and heat sink. A variety of aperture designs aredisclosed, including both same-sized apertures that form straightpassageways, and apertures that vary in size, shape and/or position toform non-axial passageways.

U.S. Pat. No. 7,009,317 discloses a motor cooling system that utilizes acooling jacket. The inner surface of the cooling jacket, which may forman interference fit with the stator, includes a series of grooves. Thegrooves along with the outer surface of the stator form a cooling ductthrough which coolant is pumped.

U.S. Pat. No. 7,633,194 discloses a system for cooling the statorlamination stack of an electric motor. The outer periphery of each ofthe laminations is defined by an array of outwardly projecting pins. Acooling jacket surrounds the stack. The outwardly projecting pinscooperate with the jacket to form a cooling space through which coolantflows.

U.S. Pat. No. 10,128,701 discloses an electric motor cooling system inwhich a plurality of axial coolant channels is integrated into thestator, preferably within the stator teeth. The axis of each of theaxial coolant channels is parallel with the cylindrical axis of thestator. A coolant manifold assembly that is integrated into the statorfluidly couples the coolant channels within the stator to the source ofcoolant.

While there are a variety of techniques that may be used to cool anelectric vehicle's motor, these techniques typically only providelimited heat withdrawal. Accordingly, what is needed is an effectivecooling system that may be used with the high power density, compactelectric motors that are commonly used in high performance electricvehicles. The present invention provides such a cooling system.

SUMMARY OF THE INVENTION

The present invention provides an electric motor cooling system that iscomprised of (i) a stator formed from a plurality of laminations, thestator including a first bulk stator portion and a second bulk statorportion, where each of the plurality of laminations includes a pluralityof slots and a plurality of stator teeth with the plurality of statorteeth alternating with the plurality of slots; (ii) a first plurality ofbulk axial coolant channels integrated into the first bulk statorportion, where the axis corresponding to each of the first plurality ofbulk axial coolant channels is parallel with the cylindrical axis of thestator, and where the first plurality of bulk axial coolant channelsterminate at a first coolant exit surface; (iii) a second plurality ofbulk axial coolant channels integrated into the second bulk statorportion, where the axis corresponding to each of the second plurality ofbulk axial coolant channels is parallel with the stator cylindricalaxis, where the second plurality of bulk axial coolant channelsterminate at a second coolant exit surface, and where the first coolantexit surface is distal from the second coolant exit surface; (iv) afirst outer stator lamination proximate to the first coolant exitsurface, the first outer stator lamination including a first pluralityof coolant channels that restrict coolant flow through the first coolantexit surface and through the first plurality of bulk axial coolantchannels; (v) a second outer stator lamination proximate to the secondcoolant exit surface, the second outer stator lamination including asecond plurality of coolant channels that restrict coolant flow throughthe second coolant exit surface and through the second plurality of bulkaxial coolant channels; (vi) a coolant manifold integrated into thestator and positioned between the first bulk stator portion and thesecond bulk stator portion, where the coolant manifold fluidly couplesthe electric motor coolant intake to the first plurality of bulk axialcoolant channels and to the second plurality of bulk axial coolantchannels; and (vii) a coolant pump that circulates a coolant through theat least one electric motor coolant intake, the coolant manifold, thefirst plurality of bulk axial coolant channels, and the second pluralityof bulk axial coolant channels.

In one aspect, the coolant flowing through the first plurality of bulkaxial coolant channels undergoes an increase in coolant velocity uponflowing through the first plurality of coolant channels, and the coolantflowing through the second plurality of bulk axial coolant channelsundergoes an increase in coolant velocity upon flowing through thesecond plurality of coolant channels.

In another aspect, the cross-sectional area corresponding to each of thefirst plurality of coolant channels is smaller than the cross-sectionalarea corresponding of each of the first plurality of bulk axial coolantchannels. Similarly, the cross-sectional area corresponding to each ofthe second plurality of coolant channels is smaller than thecross-sectional area corresponding of each of the second of bulk axialcoolant channels. Each of the first plurality of coolant channels maycompletely overlay each of the first plurality of bulk axial coolantchannels, and each of the second plurality of coolant channels maycompletely overlay each of the second plurality of bulk axial coolantchannels. Each of the first plurality of coolant channels and each ofthe second plurality of coolant channels may have a circularly-shapedcross-section.

In another aspect, (i) each of the first plurality of coolant channelspartially overlap each of the first plurality of bulk axial coolantchannels thereby creating a first plurality of overlap regions, wherethe cross-sectional area corresponding to each of the first plurality ofoverlap regions is smaller than the cross-sectional area correspondingto each of the first plurality of bulk axial coolant channels; and (ii)each of the second plurality of coolant channels partially overlap eachof the second plurality of bulk axial coolant channels thereby creatinga second plurality of overlap regions, where the cross-sectional areacorresponding to each of the second plurality of overlap regions issmaller than the cross-sectional area corresponding to each of thesecond plurality of bulk axial coolant channels.

In another aspect, the first plurality of bulk axial coolant channels ispreferably aligned with the second plurality of bulk axial coolantchannels.

In another aspect, each of the first plurality of bulk axial coolantchannels may be at least partially integrated into each of the pluralityof stator teeth corresponding to the first bulk stator portion, and eachof the second plurality of bulk axial coolant channels may be at leastpartially integrated into each of the plurality of stator teethcorresponding to the second bulk stator portion.

In another aspect, each of the first plurality of bulk axial coolantchannels and each of the second plurality of bulk axial coolant channelsmay have a cross-sectional shape selected from the group consisting ofcircularly-shaped cross-sections, rectangularly-shaped cross-sections,rectangularly-shaped cross-sections with rounded corners,elliptically-shaped cross-sections, triangularly-shaped cross-sections,and triangularly-shaped cross-sections with rounded corners.

In another aspect, the coolant flowing out of the first plurality ofcoolant channels may flow directly over a first plurality of endwindings, and the coolant flowing out of the second plurality ofchannels may flow directly over a second plurality of end windings.

In another aspect, the coolant pump may circulate the coolant through aheat exchanger.

In another aspect, the coolant may be a non-corrosive andnon-electrically conductive oil.

A further understanding of the nature and advantages of the presentinvention may be realized by reference to the remaining portions of thespecification and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

It should be understood that the accompanying figures are only meant toillustrate, not limit, the scope of the invention and should not beconsidered to be to scale. Additionally, the same reference label ondifferent figures should be understood to refer to the same component ora component of similar functionality.

FIG. 1 illustrates a portion of a stator lamination in accordance withthe prior art, this view showing the position of a plurality of axialcoolant channels integrated into the stator;

FIG. 2 provides a simplified cross-sectional view of an electric motorutilizing a cooling system as described by the prior art;

FIG. 3 illustrates a preferred configuration for the axial coolantchannels used in the bulk of the stator assembly, this figure providingan end view of a stator lamination;

FIG. 4 illustrates a portion of a stator lamination, this view showingaxial coolant channels located completely within the stator teeth;

FIG. 5 illustrates a portion of a stator lamination, this view showingaxial coolant channels located completely within the stator yoke;

FIG. 6 illustrates a portion of a stator lamination, this view showingaxial coolant channels located partially within the stator yoke andpartially within the stator teeth;

FIG. 7 illustrates a portion of an outer stator lamination, this viewshowing small coolant channels in the outer lamination overlaying thelarger coolant channels located in the underlying laminations;

FIG. 8 illustrates a portion of an outer stator lamination, this viewshowing large coolant channels in the outer lamination overlaying thecoolant channels of the underlying laminations with minimal overlap ofthe two channels;

FIG. 9 illustrates a portion of a stator assembly, this viewhighlighting the overlapping coolant channels used to create themicro-jets of coolant expelled from the stator;

FIG. 10 provides a detailed view of the overlapping coolant channelsshown in FIG. 9;

FIG. 11 provides a simplified cross-sectional view of an electric motorutilizing end stator laminations designed to restrict the flow ofcoolant ejected from the axial coolant channels integrated into thestator;

FIG. 12 provides a simplified cross-sectional view of an electric motorutilizing dual end stator laminations designed to restrict and directthe flow of coolant ejected from the axial coolant channels integratedinto the stator;

FIG. 13 provides a perspective view of a lamination stack comprising astator such as the stator assembly shown in FIG. 12;

FIG. 14 provides an end view of the middle manifold member in accordancewith the invention;

FIG. 15 provides a perspective view of the middle manifold member shownin FIG. 14;

FIG. 16 provides an end view of a transition lamination member inaccordance with the invention;

FIG. 17 provides a perspective view of the transition lamination membershown in FIG. 16;

FIG. 18 provides a perspective view of a portion of a lamination stack,this view showing the middle manifold member, left and right transitionlamination members, and a portion of the bulk lamination stack;

FIG. 19 provides the same view as that provided by FIG. 18 except forthe removal of the left transition lamination member; and

FIG. 20 provides a detailed perspective view of a portion of thelamination stack shown in FIG. 18.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

As used herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. The terms “comprises”, “comprising”, “includes”, and/or“including”, as used herein, specify the presence of stated features,integers, steps, operations, elements, and/or components, but do notpreclude the presence or addition of one or more other features,integers, steps, operations, elements, components, and/or groupsthereof. As used herein, the term “and/or” and the symbol “/” are meantto include any and all combinations of one or more of the associatedlisted items. Additionally, while the terms first, second, etc. may beused herein to describe various steps or calculations, these steps orcalculations should not be limited by these terms, rather these termsare only used to distinguish one step or calculation from another. Forexample, a first calculation could be termed a second calculation,similarly, a first step could be termed a second step, similarly, afirst component could be termed a second component, all withoutdeparting from the scope of this disclosure.

The motor and cooling systems described and illustrated herein aregenerally applicable to any high performance electric motor, andparticularly applicable to vehicles using electric traction motors,e.g., an electric vehicle (EV). In the following text, the terms“electric vehicle” and “EV” may be used interchangeably and may refer toan all-electric vehicle, a plug-in hybrid vehicle, also referred to as aPHEV, or a hybrid vehicle, also referred to as a HEV, where a hybridvehicle utilizes multiple sources of propulsion including an electricdrive system.

Electric motors typically generate heat both in the laminations due toiron core losses and in bulky conductors due to eddy currents. Themajority of the losses, however, are generated in the stator windingsdue to resistive copper losses. One common approach to removing heatfrom the stator is through the use of a coolant jacket, for example awater jacket, which is positioned around the stator laminations.Unfortunately this approach permits hot spots to develop as it does noteffectively cool the stator end-windings since the jacket is not placedin close enough proximity to the main source of heat, i.e., the statorwindings.

One technique that has been proven effective in mitigating the issue ofheat generation in the end-parts of the motor is to splash coolant,e.g., oil, on the stator end-windings and rotor end-rings. By combiningthis technique with a coolant jacket such as that described above,significant temperature drops can be achieved in an operating electricmotor. Unfortunately even this combination of cooling systems will stillallow hot spots to develop in the middle of the axial direction of themotor where neither the coolant jacket nor the coolant splashed on themotor end-parts are close enough to effectively remove heat from theseregions. Additionally, by combining two separate cooling subsystems,e.g., an outer water jacket and an oil system that includes a pump,overall system complexity is dramatically increased, leading toincreased manufacturing cost and reduced reliability.

Another technique that significantly lowers motor temperatures,especially within the stator, is the use of thin, axial cooling channelsintegrated within the stator. FIG. 1 illustrates such an approach, thisfigure showing a portion of a stator 101. Integrated within stator 101are the axial coolant channels 103. In this exemplary configuration, theaxial coolant channels 103 are located within the stator teeth 105, andpositioned between and near slots 107. Since the main heat source in thestator is the winding, locating coolant channels 103 in the teethprovides a very efficient means for removing heat from the motorassembly. Unfortunately upon exiting the axial coolant channels, thecoolant undergoes a significant decrease in velocity. As a result of thedecreased velocity, the coolant flowing out of coolant channels 103follows a gravitational flow pattern, thus preferentially cooling theend portions of the stator windings that are below the cooling channels.

FIG. 2 provides a simplified cross-sectional view of an electric motorutilizing a cooling system as described by the prior art. As shown,coolant 201 is pumped into the center, or approximate center, of thelamination stack comprising the stator 203. The coolant flows from thecenter outward towards both ends 205/206 of the stator via the axialcoolant channels 207. As the coolant exits the stator, gravitationalflow preferentially directs the coolant over the end portions of thewindings 209 that are below the exit apertures of the axial coolantchannels 207. The coolant then passes through the motor case 211 and iscollected in coolant pan 213. After exiting coolant pan 213, the coolantis pumped back into the stator using pump 215, preferably after passingthrough a filter 217. A heat exchanger 219 is preferably used towithdraw excess heat from the coolant. As FIG. 2 illustrates, the priorart approach can lead to non-uniform cooling of the end portions of thestator windings, preferentially cooling the lower end winding portions.The non-uniform cooling of the end portions of the stator windings isthe result of the coolant velocity undergoing a significant decrease asthe coolant exits the ends of the axial coolant channels. Due to thisdecrease in coolant velocity, the coolant follows a gravitational flowpattern upon exiting the coolant channels. As a result, the coolantexiting the ends of the stator does not uniformly flow over the endportions of all of the windings.

The present invention utilizes stator-integrated axial coolant channelssuch as those disclosed in co-assigned U.S. Pat. No. 10,128,701, thedisclosure of which is incorporated herein for any and all purposes. Toovercome the non-uniform cooling noted above, the present inventionutilizes stator end laminations that include coolant channels that arein fluid communication with the stator-integrated axial coolantchannels, but which are configured to restrict the flow of coolantexiting the coolant channels. As a result of this restriction, thecoolant exiting the stator-integrated coolant channels undergoes avelocity increase that directs the flow of coolant out and away from thestator, thereby uniformly impinging on the end portions of the statorwindings.

FIG. 3 illustrates a preferred configuration for the axial coolantchannels used in the stator assembly of the invention, this figureproviding an end view of a lamination 300. In the illustratedembodiment, axial coolant channels 301 are located between stator slots303, and preferably partially within the stator teeth 305. Lamination300 preferably includes one or more features 307, referred to herein askeyways, which are used to simplify alignment of the stack oflaminations during stator assembly and ensure that the coolant channels301 are aligned throughout the stack. In the preferred stator design,six keyways 307 are equidistantly spaced about the perimeter of thelamination stack. In addition to simplifying stack alignment, keyways307 can also be used during the final assembly process, for example toprovide a continuous surface for welding or bonding the stacklaminations together.

In general the size, position and shape of the axial coolant channelslocated within the bulk of the stator, e.g., coolant channels 301, areoptimized from electromagnetic, thermal and structural points of viewfor the particular application (e.g., EV traction motor), motor (e.g.,size, output, duty cycle, etc.) and cooling system (e.g., coolantcharacteristics, heat exchanger characteristics, etc.) in question. Themanufacturability of the cooling channels is also taken into account,for example insuring that the dimensions and shape of the coolingchannel lends itself to the use of a suitable manufacturing tool. Itwill be appreciated that the axial cooling channels of the invention canutilize any of a variety of shapes, e.g., generally rectangular,elliptical, triangular, circular, etc. The corners of the generallyrectangular and triangular cooling channels may be rounded or not, andif rounded the radius of curvature for the rounded corners may beoptimized. Furthermore, the axial coolant channels within the stator maybe located within the stator teeth, within the stator yoke, orpositioned such that a portion of each coolant channel lies partiallywithin the yoke and partially within the tooth. FIGS. 4-6 illustratethese coolant channels locations. In FIG. 4, a portion 400 of a statoris shown in which axial coolant channels 401 are located within eachtooth 403 of the stator. In FIG. 5, a portion 500 of a stator is shownin which axial coolant channels 501 are located completely within statoryoke 503. In FIG. 6, a portion 600 of a stator is shown in which axialcoolant channels 601 are located partially within the stator teeth 603,and partially within the stator yoke 605. When the coolant channel islocated completely or partially within the stator tooth, the innermostedge (e.g., channel edge 405) of each coolant channel is positioned at asufficient distance from the inner tooth edge (e.g., tooth edge 407) toensure the structural integrity of the tooth and in order to maintain asufficiently low magnetic saturation. While not required, preferablythere is a one-to-one correspondence between the stator teeth and theaxial coolant channels, thus simplifying manufacturing as well asinsuring uniform cooling.

An electric motor heats up with the increase of the mechanical loadingthat gives rise to the electrical current in the stator windings. Theresistive loss, P_(w), in a stator winding can be approximated by:

P _(w)=(I _(ph) ²)(R _(dc)),

where I_(ph) is the phase current and R_(dc) is the DC resistance.R_(dc) is dependent on the cross section and length of the wire used inthe windings as well as the resistivity, ρ. The resistivity, ρ, isdependent upon the temperature, T. If the temperature, T, does not varytoo much, a linear approximation such as that shown below may be used todetermine resistivity. Specifically:

ρ(T)=ρ₀[1+α(T−T ₀)],

where α is the temperature coefficient of resistivity, T₀ is a fixedreference temperature (usually room temperature), and ρ₀ is theresistivity at temperature T₀. The parameter α is an empirical parameterfitted from measurement data. In copper, α is 0.003862 K⁻¹.

The steel laminations comprising the stator assembly generate magneticcore losses that are dependent on the material properties as well as theflux density and the frequency of the power inverter supply. Theselosses, along with other motor mechanical and electrical losses, addheat to the system, leading to the rise in temperature in an operatingmotor.

Preferably the saturation of magnetic flux in the teeth remains at anoptimal level so that the electromagnetic torque of the motor ismaximized. This goal can be achieved by optimizing the stator slots,e.g., slots 303 in FIG. 3, and the axial coolant channels within thebulk of the stator laminations, e.g., coolant channels 301 in FIG. 3,together. The inventors have found that this optimization typicallyresults in a reduction in slot width, leading to decreased copper andincreased stator resistance. As shown below, the incremental increase instator resistance can be overcome by the drop in temperature, whichallows for a drop in the resistance.

From a thermal point of view, the axial coolant channels containedwithin the bulk of the stator are optimized to minimize the thermalresistance between the coolant-wetted areas of the coolant channels andthe coolant inlet section of each channel. For a given amount of heat tobe dissipated, a lower thermal resistance results in lower temperatureswithin the motor. The thermal resistance, R_(th), is related to thewetted area, A, and the heat transfer coefficient, α_(ht), through thefollowing equation:

R _(th)=1/(A·α _(ht)).

The heat, Q, extracted by a single channel can be expressed as:

Q=(T _(wall) −T _(inlet))/R _(th),

where T_(wall) and T_(inlet) represent the average temperature on thecoolant-wetted area of the channel and the average coolant temperatureat the inlet section of the cooling channel, respectively. This equationcan be rewritten as:

Q=A·α _(ht)·(T _(wall) −T _(inlet)).

Therefore the key to lowering the temperature inside the motor is tomaximize the quantity A·α_(ht), thereby minimizing the thermalresistance. The value of the heat transfer coefficient, α_(ht), isdependent on the heat transfer mechanisms occurring within the coolant,i.e., conduction and convection. Conduction results from the thermalproperties of the coolant, specifically the thermal conductivity of thecoolant. Given that the coolant flowing through the axial coolingchannels is in direct contact with the lamination stack and the copperend-windings, preferably the coolant is neither electrically conductivenor is it corrosive. In at least one embodiment of the invention, motoror transmission oil with a high dielectric strength is used as thecoolant.

The convective mechanism of heat extraction depends on the fluid motionregime within the axial coolant channels. Fluid motion within thechannels is dependent on the Reynolds number, Re, which represents theratio between the inertial and viscous forces associated with theflowing coolant and is given by:

Re=(ρ·v·D)/μ,

where ρ is the coolant density, v is the average coolant velocitymeasured on the transverse cross section of the channel, D is thehydraulic diameter and μ is the coolant dynamic viscosity. For lowReynolds numbers, typically less than 2300, the coolant regime islaminar and the main heat transfer mechanism is conduction. For highReynolds numbers, typically greater than 4000, the coolant regime isturbulent. In this case the fluctuations occurring within the coolantincrease mixing, resulting in additional heat transfer mechanism viaconvection. The coolant regime is in transition for Reynolds numbersthat are greater than 2300 and lower than 4000.

The hydraulic diameter, D, is defined as:

D=4(A _(sec) /P _(sec)),

where A_(sec) is the cross-section area and P_(sec) is the wettedperimeter of the coolant channel cross-section. As previously noted, inorder to lower motor temperature the quantity A·α_(ht) should bemaximized, preferably by maximizing both the coolant-wetted area, A, andthe heat transfer coefficient, α_(ht), which depends on the fluidregime. Expressions for the heat transfer coefficient can beconveniently written in terms of the Nusselt number, Nu, the Prandtnumber, Pr, and the ratio between the channel length, L, and hydraulicdiameter, D. Typically they take the general non-dimensional form of:

Nu=F(Pr,Re,L/D . . . ).

The Nusselt and Prandt numbers are defined as:

Nu=α _(ht)·(D/k), and

Pr=Cp·(μ/k),

where Cp is the specific heat of the cooling fluid, k is the thermalconductivity of the cooling fluid, and μ is the dynamic viscosity of thecooling fluid.

It is therefore clear from the above that there are numerous factorsthat impact the specific design of the bulk stator axial coolantchannels as applied to a specific motor; these factors include thecoolant-wetted area, A, the heat transfer coefficient, α_(ht), thetopology and dimension of the channels, and the mass flow rate. In thepreferred embodiment of the invention, the bulk stator axial coolantchannels have a generally rectangular shape with an aspect ratio betweenthe channel width and height on the order of 1:5. The preferred bulkstator axial coolant channels have dimensions of approximately 0.8 mm by4 mm.

As noted above, the outermost laminations of the stator lamination stackutilize axial coolant channels that are configured to restrict the flowof coolant passing through the stator. Restricting the flow of coolantout of the stator increases coolant velocity, thereby forming micro-jetsof coolant that are expelled from both ends of the stator. In thepreferred embodiment, the coolant exiting the stator is directed outwardat an approximately 90 degree angle relative to the planar surface ofthe outermost laminations, thus ensuring that the coolant impingesuniformly on the end windings. The size of the restricted region of thecoolant channel, while dependent upon the size of the bulk stator axialcoolant channels, is typically in the range of 0.2 mm to 1 mm indiameter, assuming a circularly-shaped region. If the restrictiveregions are elliptical or formed in some other, non-circular shape,typically they have an area in the range of 0.03 mm² to 0.8 mm².

One approach to restricting the coolant flow exiting the stator is toutilize small apertures as the exit coolant channels, where these smallapertures overlap with the underlying axial coolant channels as shown inFIG. 7. FIG. 7 provides an end view of a portion of an exemplary statorassembly 700. The underlying bulk stator axial coolant channels 701 areshown in phantom. Exit apertures 703, i.e., the coolant channels formedin the outer stator lamination 705, directly overlay the underlying bulkstator axial coolant channels 701.

While the approach of forming small coolant channels in the outermostlamination as illustrated in FIG. 7 is one approach to forming thecoolant micro-jets of the invention, this approach is difficult tomanufacture due to the small size of the outermost coolant channels,e.g., channels 703. Accordingly, in the preferred embodiment thechannels formed in the outermost stator laminations are relativelylarge, however, these channels are positioned such that they have only aminimal overlap with the underlying coolant channels, thereby formingthe desired restrictive channels. FIG. 8, which provides an end view ofa portion of an exemplary stator assembly 800, illustrates this approachto forming the coolant micro-jets. The outermost stator lamination 801includes coolant channels 803. Coolant channels 803, fabricated into theoutermost lamination 801, overlap the underlying coolant channels 805,the overlap represented by labels 807. In this figure underlying coolantchannels 805 are shown in phantom. As a result of this configuration,the coolant flowing through the bulk of the stator via channels 805 willundergo an increase in velocity as the coolant passes through therestrictive overlap region, i.e., regions 807, thereby creatingmicro-jets of coolant that are directed out and away from the stator.

Using overlapping coolant channels to create the small restrictivecoolant channels as described above relative to FIG. 8 can affect thedirection that the coolant is ejected from the stator due to the surfacereflections that the coolant undergoes as it flows through the bulkaxial coolant channels 805 and off of the affected surfaces of outermostchannels 803. As a result of this phenomenon, the direction of thecoolant exiting the stator assembly depends, in part, on the thicknessof the outermost lamination (e.g., lamination 801 in FIG. 8).

While restricting the flow of coolant as it is expelled from the statorincreases the coolant flow velocity, thereby creating coolant micro-jetsand overcoming gravitational flow and adhesion forces, it will beappreciated that the direction of coolant flow may or may not beappropriate for a particular motor design depending upon coolant channellocations, lamination thickness of the outermost lamination, etc.Accordingly in at least one embodiment, the inventors have found itdesirable to direct the coolant micro-jets in order to effectively coolthe stator end windings. In order to direct the flow of coolant from thestator assembly in the desired direction, which in at least oneembodiment is in a direction generally perpendicular to the surfaceplane of the outermost lamination, the inventors have found that the useof two outer laminations is preferred. FIG. 9 provides a perspectiveview of a stator lamination stack 900 that uses a pair of outerlaminations 901 and 903 to both restrict coolant flow and direct theexiting coolant micro-jets to the desired direction. In FIG. 9, onlyportions of outer laminations 901 and 903 are visible in order to betterillustrate the invention. In this embodiment, the stack of laminations905 comprising portion 907 of the stator, i.e., the bulk of the stator,utilize axial coolant channels 909. Coolant channels 909 utilize thesame channel design as channels 301 shown in FIG. 3. The first outerlamination 901, i.e., the lamination that is directly adjacent to thestack of laminations 905, utilizes large coolant channels 911. Asdescribed above relative to FIG. 8, coolant channels 911 only overlapunderlying channels 909 by a small amount in order to create restrictivechannels 913. The second outer lamination 903, i.e., the outermostlamination comprising the stator, includes a plurality of channels 915.In this embodiment, lamination 903 includes the same coolant channelpattern and coolant channel shape/size as the laminations 905 that formthe bulk 907 of the stator. As such, channels 915 overlap channels 911.The combination of lamination 901, which restricts coolant flow, andlamination 903, which directs coolant flow, generates a series ofmicro-jets that are expelled from the end of stator stack 900 in anapproximately 90 degree angle relative to the planar surface oflamination 903. FIG. 10 provides a more detailed view of the overlappingcoolant channels included in stack 905, first outer lamination 901 andsecond outer lamination 903.

It should be understood that the use of dual outer laminations asdescribed above relative to FIG. 9 may be accomplished using differentlyshaped and sized axial coolant channels. However the design providedabove also optimizes motor manufacturability by limiting the number ofdifferent lamination designs that are required to fabricate the stator,thus optimizing manufacturing, i.e., stamping, efficiency. For example,channels 911 serve dual purposes. The lower portion of channels 911 aredesigned to overlap stator channels 915 to the degree necessary torestrict the flow of coolant as desired while the upper portion ofchannels 911 are designed to provide passageways for the coolant toenter the stator channels as described in detail below. Additionally, inthis embodiment the same lamination design is used for both the bulklaminations 905 and for the outermost lamination 903, thus allowing thisdesign to also serve dual purposes.

In order to achieve optimal heat removal preferably the coolant (e.g., anon-corrosive, non-electrically conductive oil) is fed into the centerof the lamination stack, rather than into one end of the stack. Feedinginto the center of the stack allows shorter cooling channels, i.e., leftand right portions of the stack rather than extending throughout theentire stack, thus providing higher average heat transfer coefficientsand improved cooling. Additionally, feeding into the stack center allowscooling to start in the middle of the stack where heat is trapped andhot spots typically occur.

FIGS. 11 and 12 provide simplified cross-sectional views of exemplaryelectric motors utilizing cooling systems as described herein. Theembodiment shown in FIG. 11 uses coolant channels within a singleoutermost lamination, such as those shown in FIG. 8, to create thecoolant micro-jets formed when the coolant is ejected from the axialcoolant channels integrated into the stator. The embodiment shown inFIG. 12 uses coolant channels within a pair of outermost laminations,such as those shown in FIGS. 9 and 10, to create and direct the coolantmicro-jets formed when the coolant is ejected from the axial coolantchannels integrated into the stator.

In FIG. 11, coolant 1101 is pumped into the center, or approximatecenter, of the lamination stack comprising the stator 1103. The coolantflows from the center outward in directions 1105 towards ends 1107 ofthe stator via the bulk stator axial coolant channels 1109. In at leastone embodiment bulk stator axial coolant channels 1109 are similar tocoolant channels 805 shown in FIG. 8. Upon exiting the bulk axialcoolant channels 1109, the coolant passes through coolant channels 1111formed in stator end laminations 1113. As previously noted in describingFIG. 8, due to the minimal overlap between channels 1109 and 1111, i.e.,regions 1115, the restriction causes a velocity increase in the coolantejected from the stator. The increase in ejected coolant velocityovercomes the gravitational flow pattern, thus helping to ensure thatthe ejected coolant impinges on the stator end windings (not shown forclarity). After cooling the end windings, the coolant passes through themotor case 1117 and is collected in coolant pan 1119. After passingthrough a heat exchanger 1121, and preferably after passing through afilter 1123, the coolant is pumped back into the stator using pump 1125.

The embodiment shown in FIG. 12 is similar to that shown in FIG. 11,except that this embodiment includes dual outer laminations 1201 and1203 as described above relative to FIGS. 9 and 10. As in the priorembodiment, the coolant 1101 exiting from the bulk stator axial coolantchannels 1109 first pass through coolant channels 1205 formed in statorend laminations 1201, where laminations 1201 and channels 1205 aresimilar to, or the same as, laminations 1113 and channels 1111. Due tothe minimal overlap between channels 1109 and 1205, i.e., regions 1207,the restriction causes a velocity increase in the coolant ejected fromthe stator. Next the coolant passes through channels 1209 included insecond end laminations 1203. Preferably channels 1209 are identical tochannels 1109 used in the bulk laminations. As described above, there isminimal overlap, i.e., regions 1211, between channels 1209 fabricatedinto end laminations 1203 and channels 1205 fabricated into endlaminations 1201. Channels 1209 in end laminations 1203 direct the flowof the micro-jets created by channels 1205 in the first end laminations1201. The combination of the two channels 1205 and 1209 in the dualouter laminations 1201/1203 both generate and direct the coolantmicro-jets which impinge on the end windings. As in the priorembodiment, in this preferred embodiment after cooling the end windings,coolant 1101 passes through the motor case 1117, is collected in coolantpan 1119, passes through filter 1123, and is pumped by pump 1125 throughheat exchanger 1121.

FIG. 13 provides a perspective view of a lamination stack 1300, such asthe lamination stack shown in FIG. 12, this assembly including the dualouter laminations such as those described above relative to FIGS. 9 and10. It will be appreciated that in this simplified view the individuallaminates comprising the lamination stack are not individually visible.Additionally the stator windings are not shown in this figure, thusallowing a better view of the individual stator teeth 1301. It should beunderstood that since this view does not include a cut-away of theoutermost laminations, the bulk axial coolant channels, e.g., coolantchannels 909 in FIG. 9, are not visible. In this figure only thechannels 1303 fabricated into the outermost lamination 1305 are visible.It will be appreciated that the design and manufacture of the stator,with the exception of the axial cooling channels in the bulk and theouter laminations as well as the coolant manifold design describedherein, is well known and therefore a detailed description will not beprovided. Additionally, it should be understood that the coolantmanifold described below and illustrated in FIG. 13 is equallyapplicable to a stator assembly utilizing a single micro-jet formingouter lamination such as the designs shown in FIGS. 7 and 8 anddescribed above.

In general, stator assembly 1300 is comprised of a stack of plates,typically referred to as laminations, where each plate is electricallyinsulated from the adjacent plate(s). The plates are normally stamped orotherwise fabricated from a single sheet of material (e.g., steel). Toachieve electrical isolation, both surfaces of each plate are coatedwith an electrically insulating layer. The electrically insulatingcoating may be applied before or after the fabrication of the plate,e.g., before or after stamping. Since each plate includes one or morelayers of an electrically insulating material, after coating the plateis generally referred to as a laminate or lamination, and the stack ofplates is generally referred to as a lamination stack. After stackassembly, the windings are disposed about the stator teeth.

Incorporated into the stator, and located between the left portion 1307and the right portion 1309 of the lamination stack, is a coolantmanifold 1311. The coolant manifold 1311 is coupled to the statorcoolant intake, e.g., coolant intake 1127 shown in FIGS. 11 and 12. Thecoolant is pumped through intake 1127 and into manifold 1311, themanifold then distributing the coolant to all of the bulk axial coolantchannels which are not visible in this figure, but are similar to or thesame as coolant channels 301 shown in FIG. 3. By locating the manifoldat or near the center of the stator, the coolant is pumped outwardlyfrom the stator center to both ends of the stator assembly. Manifold1311 is coupled to, and sealed to, intake 1127 such that the coolantthat is pumped through intake 1127 flows about the entire perimeter ofmanifold 1311. By sealing the intake to the manifold, the coolant isforced through the manifold into all of the bulk axial cooling channels.

In the preferred stator design, each of the laminations comprising thestator assembly includes at least one keyway 1313. More preferably, andas shown, each of the stator laminations includes a plurality of keywaysequidistantly spaced about the perimeter of the laminations. Keyways1313 serve multiple purposes. First, the keyways 1313 allow the statorlaminations to be easily assembled while maintaining the alignment ofthe axial coolant channels in order to ensure that the coolant flowsuninterrupted through the channels. Second, keyways 1313 improve therigidity of the entire lamination stack 1300. Third, keyways 1313provide a convenient region for welding or bonding the stack together.

FIGS. 14 and 15 provide end and perspective views, respectively, of themiddle manifold member 1400 that is located in the center, orapproximate center, of manifold 1311. Middle manifold member 1400 ispreferably comprised of multiple individual, identical, laminations. Theouter diameter 1401 of member 1400 is smaller than the outside diameterof the remaining stator laminations. Preferably member 1400 does,however, include keyways 1313. Although not required, preferably member1400 also includes the same pattern of bulk axial coolant channels,e.g., channels 301, as used throughout the stator. The axial coolantchannels included in the middle manifold member 1400 ensure that thismember does not develop hot spots.

FIGS. 16 and 17 provide end and perspective views, respectively, of thetransition members 1600 that are located on either side of the middlemanifold member 1400. FIG. 18 provides a perspective view of a portionof lamination stack 1300, this view showing right lamination portion1309, middle manifold member 1400, and left and right transitionlamination members 1600A and 1600B, respectively. FIG. 19 provides thesame view as that provided by FIG. 18 except for the removal of lefttransition lamination member 1600A. FIG. 20 provides a detailedperspective view of a portion of the lamination stack shown in FIG. 18.

Transition lamination members 1600A and 1600B include the same patternand configuration of axial coolant channels 301 as used in the bulk leftand right lamination stack portions 1307 and 1309. Channels 301 are influid communication with coolant distribution channels 1601, i.e., thereis no barrier between channels 301 and channels 1601. As shown in FIG.19, a portion 1901 of each coolant distribution channel 1601 extendsbeyond the perimeter of middle manifold member 1400. As a result of thisdesign, coolant flowing into manifold 1311 flows into the distributionchannels 1601 via portions 1901, and then into axial coolant channels301.

Due to the inclusion of keyways 1311 on middle manifold member 1400,preferably the transition lamination members include coolant cut-outs1603 that ensure that the coolant distribution channels 1601A-1601C inthat region of the member receive coolant. Therefore cut-outs 1603 areconfigured to ensure that the keyways 1313 fabricated into the middlemanifold member 1400 do not interfere with the flow of coolant into thecoolant distribution channels fabricated into the transition laminationmembers, and thus do not interfere with the flow of coolant into theaxial coolant channels 301. Cut-outs 1603 also allow the transitionmembers to maintain a continuous, uninterrupted outer perimeter whichinclude keyways 1313.

Middle manifold member 1400 as well as the transition lamination members1600 are preferably fabricated using the same process, e.g., stamping,as the lamination members used throughout the lamination stack.

Systems and methods have been described in general terms as an aid tounderstanding details of the invention. In some instances, well-knownstructures, materials, and/or operations have not been specificallyshown or described in detail to avoid obscuring aspects of theinvention. In other instances, specific details have been given in orderto provide a thorough understanding of the invention. One skilled in therelevant art will recognize that the invention may be embodied in otherspecific forms, for example to adapt to a particular system or apparatusor situation or material or component, without departing from the spiritor essential characteristics thereof. Therefore the disclosures anddescriptions herein are intended to be illustrative, but not limiting,of the scope of the invention.

What is claimed is:
 1. An electric motor cooling system, comprising: astator comprising a plurality of laminations, said stator comprising afirst bulk stator portion and a second bulk stator portion, wherein eachof said plurality of laminations includes a plurality of slots and aplurality of stator teeth, wherein said plurality of stator teethalternate with said plurality of slots; a first plurality of bulk axialcoolant channels integrated into said first bulk stator portion, whereinan axis corresponding to each of said first plurality of bulk axialcoolant channels is parallel with a cylindrical axis corresponding tosaid stator, and wherein said first plurality of bulk axial coolantchannels terminate at a first coolant exit surface corresponding to saidfirst bulk stator portion; a second plurality of bulk axial coolantchannels integrated into said second bulk stator portion, wherein anaxis corresponding to each of said second plurality of bulk axialcoolant channels is parallel with said cylindrical axis corresponding tosaid stator, wherein said second plurality of bulk axial coolantchannels terminate at a second coolant exit surface corresponding tosaid second bulk stator portion, and wherein said first coolant exitsurface is distal from said second coolant exit surface; a first outerstator lamination proximate to said first coolant exit surface, saidfirst outer stator lamination comprising a first plurality of coolantchannels, said first plurality of coolant channels restricting coolantflow through said first coolant exit surface and through said firstplurality of bulk axial coolant channels; a second outer statorlamination proximate to said second coolant exit surface, said secondouter stator lamination comprising a second plurality of coolantchannels, said second plurality of coolant channels restricting coolantflow through said second coolant exit surface and through said secondplurality of bulk axial coolant channels; a coolant manifold integratedinto said stator and positioned between said first bulk stator portionand said second bulk stator portion, wherein said coolant manifoldfluidly couples an electric motor coolant intake to said first pluralityof bulk axial coolant channels and to said second plurality of bulkaxial coolant channels; and a coolant pump, wherein said coolant pumpcirculates a coolant through said at least one electric motor coolantintake, said coolant manifold, said first plurality of bulk axialcoolant channels, and said second plurality of bulk axial coolantchannels.
 2. The electric motor cooling system of claim 1, wherein saidcoolant flowing through said first plurality of bulk axial coolantchannels undergoes an increase in coolant velocity upon flowing throughsaid first plurality of coolant channels, and wherein said coolantflowing through said second plurality of bulk axial coolant channelsundergoes an increase in coolant velocity upon flowing through saidsecond plurality of coolant channels.
 3. The electric motor coolingsystem of claim 1, wherein: a first cross-sectional area correspondingto each of said first plurality of coolant channels is smaller than asecond cross-sectional area corresponding to each of said firstplurality of bulk axial coolant channels; and a third cross-sectionalarea corresponding to each of said second plurality of coolant channelsis smaller than a fourth cross-sectional area corresponding to each ofsaid second plurality of bulk axial coolant channels.
 4. The electricmotor cooling system of claim 3, wherein: each of said first pluralityof coolant channels completely overlays each of said first plurality ofbulk axial coolant channels; and each of said second plurality ofcoolant channels completely overlays each of said second plurality ofbulk axial coolant channels.
 5. The electric motor cooling system ofclaim 4, wherein each of said first plurality of coolant channels andeach of said second plurality of coolant channels has acircularly-shaped cross-section.
 6. The electric motor cooling system ofclaim 1, wherein: each of said first plurality of coolant channelspartially overlaps each of said first plurality of bulk axial coolantchannels to create a first plurality of overlap regions, wherein a firstcross-sectional area corresponding to each of said first plurality ofoverlap regions is smaller than a second cross-sectional areacorresponding to each of said first plurality of bulk axial coolantchannels; and each of said second plurality of coolant channelspartially overlaps each of said second plurality of bulk axial coolantchannels to create a second plurality of overlap regions, wherein athird cross-sectional area corresponding to each of said secondplurality of overlap regions is smaller than a fourth cross-sectionalarea corresponding to each of said second plurality of bulk axialcoolant channels.
 7. The electric motor cooling system of claim 1,wherein said first plurality of bulk axial coolant channels is alignedwith said second plurality of bulk axial coolant channels.
 8. Theelectric motor cooling system of claim 1, wherein each of said firstplurality of bulk axial coolant channels is at least partiallyintegrated into each of said plurality of stator teeth corresponding tosaid first bulk stator portion, and wherein each of said secondplurality of bulk axial coolant channels is at least partiallyintegrated into each of said plurality of stator teeth corresponding tosaid second bulk stator portion.
 9. The electric motor cooling system ofclaim 1, wherein each of said first plurality of bulk axial coolantchannels and each of said second plurality of bulk axial coolantchannels has a cross-sectional shape selected from the group consistingof circularly-shaped cross-sections, rectangularly-shapedcross-sections, rectangularly-shaped cross-sections with roundedcorners, elliptically-shaped cross-sections, triangularly-shapedcross-sections, and triangularly-shaped cross-sections with roundedcorners.
 10. The electric motor cooling system of claim 1, wherein saidcoolant flowing out of said first plurality of coolant channels flowsdirectly over a first plurality of end windings, and wherein saidcoolant flowing out of said second plurality of channels flows directlyover a second plurality of end windings.
 11. The electric motor coolingsystem of claim 1, wherein said coolant pump circulates said coolantthrough a heat exchanger.
 12. The electric motor cooling system of claim1, said coolant comprising an oil, wherein said oil is non-corrosive andnon-electrically conductive.